You would definitely be able to change your ATL-FRA flight to F for free (since that is what you paid for after all). But you can't waitlist..you just have to keep calling back and checking. I'm pretty sure that once you're at the airport you'd be out of luck as you will be presenting a business class ticket to LH at that time..they won't be able to tell that you actually paid for first. So hopefully this can be resolved at least 24 hours in advance of the flight through UA reservations.